The present invention relates to a process for isolating enantiomer components from a mixture of enantiomers, as well as to separating a racemate into its enantiomeric components. Separated, single enantiomers are extremely important in certain fields of use, since they contain desired properties whereas their enantiomer pair may contain undesirable properties.
Isolation of enantiomers from a mixture of enantiomers is typically difficult because the enantiomers generally have identical physical properties, such as melting and boiling points, or other such properties typically used for separation. Moreover they tend to crystallize as racemic crystals rather than as a conglomerate consisting of a mixture of pure enantiomer crystals which would be separable by preferential crystallization. Thus, a common way today to obtain enantiomers is not through isolating individual enantiomers from a mixture, but rather through asymmetric synthesis of the enantiomer.
Techniques for isolating enantiomers in use today include various embodiments of chromatography, such as simulated moving bed chromatography (SMB). Chromatography-based methods, however, to date are not capable of isolating some enantiomers and/or cannot isolate some enantiomers economically in commercial quantities.
Various crystallization methods have also been proposed for separating enantiomers from a mixture, including preferential crystallization, co-crystallization and emulsion-crystallization. C.f. EP 0 548 028 A1; WO 97/32644; EP 0 838 448 A1; U.S. Pat. No. 5,898,075. While these methods overcome many of the shortcomings in crystallization, they also have some shortcomings. Preferential crystallization works only with racemates forming conglomerates. Furthermore, it is difficult to conduct with many conglomerates, since the systems tolerate only a small degree of super-saturation before spontaneous nucleation of the unwanted isomer occurs.
In co-crystallization, the yields of the enantiomer to be isolated and its co-crystallization agent are often poor ( less than 95%), and it is normally difficult to recover the other enantiomer of the mixture in pure form. Furthermore, it can be difficult to identify a suitable co-crystallization agent that is inexpensive and readily accessible, and which enables crystallization of the desired enantiomer in high yield. Through emulsion crystallization some racemates forming racemic crystals (i.e. conglomerates and especially racemic compounds; see R. A. Sheldon, Chirotechnology, Marcel Dekker, Inc. 1993 (p. 174) for general definitions of terminology) can be separated, however the majority of the racemic crystals forming racemates cannot be separated even by normal emulsion crystallization.
It has now been found that the aforementioned problem can be avoided through the use of two co-crystallization agents which selectively form co-crystals with the (R) and (S) enantiomers of a mixture of enantiomers. The co-crystals so-formed can then be readily isolated and subsequently treated to yield the desired enantiomers.
In a second aspect of the invention, one or more enantiomers can be isolated from a mixture of enantiomers through co-crystallization from an emulsion. Use of an emulsion lends additional benefits characteristic of emulsion crystallization to the present invention.
The present invention provides a process for isolating enantiomer components from a mixture of enantiomers through co-crystallization comprising the steps of (a) forming a solution comprising the mixture of enantiomers (R) and (S) and co-crystallization agents C1 and C2, wherein C1 and C2 are chiral or achiral, with the proviso that at least one of C1 and C2 is chiral and C1 and C2 do not form an enantiomeric pair, whereby C1 forms a co-crystal with (R) and C2 forms a co-crystal with (S); (b) super-saturating the solution in C1*(R) and C2 *(S); (c) inducing crystallization of co-crystals of C1*(R) and C2*(S); and (d) isolating the C1*(R) co-crystals and C2*(S) co-crystals.
A second aspect of the present invention provides a process for isolating one or more enantiomers from a mixture of enantiomers through co-crystallization from an emulsion comprising the steps of (a) forming an emulsion of organic liquid droplets in a continuous water phase, which emulsion contains the mixture of enantiomers and a co-crystallization agent for each enantiomer to be isolated, wherein the co-crystallization agents are chiral or achiral, with the proviso that at least one co-crystallization agent is chiral, whereby the co-crystallization agent forms a co-crystal with its corresponding enantiomer; (b) super-saturating the emulsion in (co-crystallization agent) *(enantiomer); (c) inducing crystallization of co-crystals of (co-crystallization agent) *(enantiomer), whereby crystallization takes place in the water phase; and (d) isolating the co-crystals of (co-crystallization agent) *(enantiomer).
Crystallization processes are known and need not be described in detail here. Their basic premise is that a solution is formed containing the desired substance, the solution is supersaturated by conventional techniques such as cooling of the solution, and then crystallization of the desired substance is induced, either spontaneously or by seeding with seed crystals of the desired substance. The present extends this technology through the judicious choice of co-crystallization agents which will form co-crystals with the enantiomers of a mixture of enantiomers. The solution accordingly contains the enantiomers and the co-crystallization agents, is super-saturated, and then crystallization of co-crystals of the enantiomers and co-crystallization agents is induced.
Co-crystals of the co-crystallization agents and the enantiomers are indicated in the present invention according to the convention xe2x80x98(co-crystallization agent) *(enantiomer)xe2x80x99. In a typical embodiment of the invention, two co-crystallization agents, C1 and C2, will be employed. They will, accordingly, selectively form co-crystals with the (R) and (S) enantiomers of the mixture of enantiomers, as indicated by xe2x80x98C1*(R)xe2x80x99 and xe2x80x98C2*(S)xe2x80x99. All stoichiometries are intended to be covered with this nomenclature, i.e., xe2x80x98C1*(R)xe2x80x99 should be understood to include 1 C1*(R); 2 C1*(R); 1 C1*2(R); 2 C1*3(R); etc.
(R) and (S) may be present in the enantiomeric mixture in any ratio, including a 50/50 ratio, i.e. as a racemate. The mixture may comprise more than one pair of (R) and (S) enantiomers. The enantiomers can be bases in which case the co-crystallization agents typically will be acids. Or, the enantiomers can be acids in which case the co-crystallization agents typically will be bases. Bases will typically be amines. Alternatively, the enantiomers and/or co-crystallization agents can be neutral co-crystal-forming compounds.
The enantiomers can be pharmaceutical or agrochemical substances, fragrances, food additives, chemical intermediates or the like.
Co-crystallization agents are compounds that selectively form co-crystals with the (R) and (S) enantiomers. Co-crystallization agents may be either chiral or achiral, though at least one of them must be chiral. Preferably, both are chiral. Co-crystallization agents can not form an enantiomeric pair as this could lead to formation of a racemic co-crystal consisting of C1*C2*R*S.
An exception to the limitations on the co-crystallization agents applies to the case of co-crystallization from an emulsion (later described). In this case, the process of the present invention can be carried out using one co-crystallization agent to isolate a single enantiomer (R) or (S) from the mixture of enantiomers. The co-crystallization agent must be chiral. Where two co-crystallization agents are used to isolate both enantiomers, the co-crystallization agents can be chiral or achiral, with the proviso that at least one is chiral. In the case both are chiral they can form an enantiomeric pair. Typically, however, the conditions of the previous paragraph will also apply to emulsion crystallization.
The co-crystallization agents used in the present invention are preferably chosen according to the following guidelines:
Co-crystals C1*(R) and C2*(S) should be less soluble than all other crystals that may form (e.g. C1*(R)*(S); (R)*(S); C1*C2*(R)*(S); etc), under the conditions at which crystallization takes place. In the case of emulsion crystallization, however, their solubilities can be somewhat higher than those of other crystals, since control over what crystallizes is possible through seeding with the desired co-crystal (see also further discussion on emulsion crystallization);
Concentrations of the co-crystallization agents and crystallization conditions are selected such that C1*(R) and C2*(S) can either be 1) alternately co-crystallized according to preferential crystallization: or (especially in the case of emulsion crystallization) 2) simultaneously crystallized, when differences in crystal size or shape between C1*(R) and C2*(S) allow separation by sieving or sedimentation;
At least one of the co-crystallization agents is chiral.
In addition, C1 and C2 may each comprise a family of two or more co-crystallization agents. Each member of the family typically have a common base structure. The features defined above for C1 and C2 will apply to each member of the family.
Inducing crystallization (step (c)) can be carried out either by seeding with co-crystals of (co-crystallization agent)*(enantiomer) (i.e. C1*(R), C2*(S) etc.) or can occur without seeding (i.e. by spontaneous crystallization). Seeding can be carried out consecutively or (especially in the case of emulsion crystallization) simultaneously. In the case of consecutive seeding, the solution is first seeded with co-crystals of C1*(R) to induce crystallization of C1*(R) co-crystals, which co-crystals are then isolated from solution, and then the solution is seeded with co-crystals of C2*(S) to induce crystallization of C2*(S) co-crystals, which co-crystals are then isolated from solution. Or, this order of seeding can be reversed.
In the case of simultaneous seeding, differences in crystal size or shape between C1*(R) and C2*(S) allow their separation by sieving or sedimentation.
Prior to seeding, it is desirable that the super-saturated solution (or emulsion) contains no seed crystals of substances apart from those that are intended to be seeded. Any seeds present can be dissolved by ultrasound or heating, with such dissolving of seeds hereinafter referred to as xe2x80x9chomogenisationxe2x80x9d.
In a preferred embodiment of the invention, significant improvements in yields can be obtained if the crystallization process of the present invention is carried out with recycle of solution (or emulsion). This necessitates, in effect, that the solution (or emulsion) is replenished with the mixture of enantiomers and co-crystallization agents and then steps (a)-(d) are repeated. The order in which replenishing with the mixture of enantiomers and co-crystallization agents and steps (a)-(d) are repeated can vary. The mixture of enantiomers and co-crystallization agents can be replenished together, in a single step, following step (d). Or, the mixture of enantiomers and co-crystallization agents can be replenished after isolation of C1*(R) co-crystals and again after isolation of C2*(S) co-crystals. Other sequences are possible, as will be known to one skilled in the art.
Recycle can be applied to a variation of the second aspect of the present invention which brings about particular advantages. In this variation, both enantiomers are isolated from the mixture using two separate emulsions: a first emulsion is formed for super-saturating, inducing crystallization and isolating co-crystals of the first enantiomer and a second emulsion is formed for super-saturating, inducing crystallization and isolating co-crystals of the second enantiomer in steps (b), (c) and (d) of the process. Then, following isolation of co-crystals in step (d), a further step (e) is employed wherein the left-over first emulsion is recycled for use as the second emulsion and the left-over second emulsion is recycled for use as the first emulsion in step (a).
By carrying out a recycle, several advantages are obtained. The co-crystallization agents, which are often costly, can be re-used. The process can be carried out continuously. Yield of co-crystal after each crystallization step need not be as high as is required in classical (non-emulsion) co-crystallization (where only one enantiomer is recovered by co-crystallization with a chiral co-crystallization agent), whereas overall yield with recycle can be significantly higher. This enables the use of a variety of co-crystallization agents in the present invention, which in turn enables a wide range of mixtures of enantiomers to be resolved economically. Also, the present process isolates both enantiomers, whereas classical co-crystallization only one. The xe2x80x98otherxe2x80x99 enantiomer can have value as an intermediate, as a co-crystallization agent for another separation process or to racemize it into the xe2x80x98desiredxe2x80x99 enantiomer in a separate step. Racemization in the crystallization solution is an often employed process. However, the racemization conditions are restricted to those possible in the given crystallization solvent, in the presence of the desired enantiomer and its co-crystallization agent, and the like.
The desired enantiomer can be isolated from the co-crystallization agent using standard techniques, such as extraction of a co-crystallizing acid by an alkaline solution or vice versa.
Particular advantages are gained if the crystallization is carried out from an emulsion. Emulsions allow, for example, crystallization at constant, low temperature with little or no spontaneous nucleation, extreme super-saturation, slow crystal growth, very regular crystal shapes, narrow distribution of crystal size and high purities.
Emulsions also facilitate the possibility of simultaneous seeding of co-crystals. Accordingly, the solution can be (in a preferred embodiment is) seeded in step (c) simultaneously with seed co-crystals of C1*(R) and C2*(S), and the isolation of step (d) is carried out by separating co-crystals of C1*(R) from co-crystals of C2*(S) by sieving or sedimentation
Emulsion crystallization can be used to isolate one or more enantiomers from the mixture of enantioners.
Emulsions are, by definition, xe2x80x9cdropletsxe2x80x9d dispersed in a xe2x80x9ccontinuous phasexe2x80x9d. In the present invention, the droplets are organic liquid droplets and the continuous phase is a water phase. The emulsion optionally contains additives such as surfactants and dispersants, known in the art, for assisting formation and stabilization of the emulsion, and for facilitating the transport of the co-crystallization agent and/or enantiomer out of the organic liquid droplets and into the water phase, where crystallization takes place on a crystal surface (i.e. either the seed crystal or spontaneously formed crystal). Such surfactants and/or dispersants will be chosen according to the nature of the emulsion, and can be nonionic, anionic and/or cationic. The surface active agent will normally be present in an amount of 0.01-30 w/w % , preferably 0.1-20 w/w %.
The droplets typically vary in diameter from approximately 0.05 to 80 xcexcm. Droplets with diameter in the range of 0.3 to 80 xcexcm are known as xe2x80x9cmacrodropletsxe2x80x9d, and the emulsions as xe2x80x9cmacroemulsionsxe2x80x9d. Droplets with diameter in the range of 0.05 to 0.3 xcexcm are known as xe2x80x9cmicrodropletsxe2x80x9d, and the emulsions as xe2x80x9cmicroemulsionsxe2x80x9d. For the sake of simplicity, the terms xe2x80x9cdropletsxe2x80x9d and xe2x80x9cemulsionsxe2x80x9d as used herein also encompass both macro- and microdroplets and macro- and microemulsions.
The organic liquid phase of the droplet will be water insoluble. xe2x80x98Water insolublexe2x80x99 in this context means anything less than water miscible, though in most cases the organic liquid phase will mix with water in an amount not more than 30% w/w at the temperature at which crystallisation takes place.
In the emulsion of the present invention, at least one of the co-crystallization agents or enantiomers will be present primarily in the organic liquid droplets of the emulsion. xe2x80x98Primarilyxe2x80x99 means in this sense its concentration in the organic liquid droplets is at least 20% higher than in the water phase (the relative solubility can e.g. be determined at least approximately in the absence of the tenside, determining the equilibrium distribution).
The water may further contain a buffering agent, such as sodium acetate and acetic acid, for maintaining pH of the emulsion at a desired level, antifreezing agents and solubility adjusting agents, as is known in the art.
Emulsions according to the invention can be formed using techniques known in the art. A suitable example for carrying out the invention, which is not intended to limit the cope of the present invention in any way, is as follows (Synperonic NP10 is a nonlyphenol surfactant, ethoxylated with 10 mol ethyleneoxide, ICI PLC, England; Soprophor FL is a tristyrylphenol derivative, Rhodia, France):